I am using drupal 7.56 + civicrm 4.7.25 I have a Paypal account and I configured a Payment Processor for test with the REST API credentials from https://developer.paypal.com
REST API Sandbox account becomes PP User name REST API Secret becomes PP Password REST API Client ID becomes PP Signature
This configuration does not work with error message :
Payment approval failed with message :Security error Security header is not valid
Where can I find API credentials required by Payment Processor configuration ? And more generally could someone tell a detailed end to end Paypal Express Checkout Civicrm integration ?